Reliquefaction of boil off gas

ABSTRACT

Method of reliquefying gas resulting from the evaporation of a ship&#39;&#39;s cargo of liquefied gas which utilizes a compression refrigerator having two refrigerant loops in one of which the compressed refrigerant is expanded in a turbine and in the other of which it is expanded without the performance of external work. The expansion turbine and a gas turbine which can be fueled by the gas provide the power for compressing the refrigerant, and the gas is reliquefied by heat exchange with cold expanded refrigerant in the second loop. In one embodiment open loops are used with the gas providing the refrigerant.

United States Paten [191 Jones Nov. 18, 1975 [73] Assignee: Petrocarbon Developments Limited,

Manchester, England [22] Filed: July 30, 1974 [21] Appl. No.: 493,094

Related US. Application Data [63] Continuation-impart of Ser. No. 389,115, Aug. 17,

1973, abandoned.

[30] Foreign Application Priority Data Apr. 17, 1973 United Kingdom 18484/73 July 27, 1973 United Kingdom 35796/73 [52] US. Cl. 62/7; 62/54; 62/240; 60/651; 60/675 [51] Int. Cl. F25B 19/00 [58] Field of Search 62/7, 50, 54, 55, 240, 62/514; 60/651, 675; 114/74 A [56] References Cited UNITED STATES PATENTS 2,940,268 6/1940 Morrison 62/7 3,347,055 10/1967 Blanchard et a1 62/54 3,375,675 4/1968 Trepp et a1. 62/54 3,611,740 10/1971 Giger 62/514 3.613.387 10/1971 Collins 62/514 3,720,057 3/1973 Arenson 62/52 Primary Examiner-William E. Wayner Assistant Examiner-Ronald C. Capossela Attorney, Agent, or Firm-Browdy and Neimark [57] ABSTRACT Method of reliquefying gas resulting from the evaporation of a ships cargo of liquefied gas which utilizes a compression refrigerator having two refrigerant loops in one of which the compressed refrigerant is expanded in a turbine and in the other of which it is expanded without the performance of external work. The expansion turbine and a gas turbine which can be fueled by the gas provide the power for compressing the refrigerant, and the gas is reliquefied by heat exchange with cold expanded refrigerant in the second loop. In one embodiment open loops are used with the gas providing the refrigerant.

28 Claims, 4 Drawing Figures EXHAUST U.S. Patent Nov. 18, 1975 Sheet 3 of4 I H/APJETJS FHOMCARGO LNG RELIQUEFACTION OF BOIL-OFF GAS CROSS REFERENCE TO RELATED APPLICATION The present application is a continuation-in-part of US. Ser. No. 389,1 l5, filed Aug. 17, 1973, now abandoned.

FIELD OF THE INVENTION This invention relates to the reliquefaction of boil-off gas.

BACKGROUND OF THE INVENTION In ocean tankers carrying cargoes of liquid gas a portion of the liquid normally amounting to approximately 0.1 0.25% per day in the case of liquefied natural gas evaporates in the course of the voyage as a result of heat leakage through the insulation This evaporated liquid is known as boil-off gas.

Cargoes of liquefied gas which are to be transported by tanker are frequently stored for a short period of time at the export terminal in insulated tanks prior to loading on board the tanker. These land based tanks at the export terminal are usually maintained at just above atmospheric pressure, with the liquefied gas at its saturated bubble point temperature. The liquid is then pumped to a higher pressure and passed along insulated lines into the tankers cargo compartment. The pumping and the heat leakage through the insulation results in a loaded cargo which is at a higher energy level than the liquid in the land based storage tanks. The loaded cargo will be at a slightly higher pressure and bubble point temperature than in the land based tanks.

Before this cargo is unloaded at the import terminal it is desirable to reduce theenergy level of the cargo to that of the liquid in the land storage tanks. The reason for reducing the energy level and pressure of the cargo is to reduce the quantity of vapor generated during the unloading operation as a result of expanding the liquid cargo to the lower pressure. This is because the import terminal may not have sufficient vapor handling equipment to handle this additional gas or there may not be sufficient immediate gas demand to allow the additional vapor to be passed to the distribution system. In present practice, the reduction in energy level of the loaded cargo generally takes place on the tanker whilst it is steaming from export to import terminal. This is normally done by reducing the gas pressure above the cargo liquid surface by sucking off gas. This pressure reduction results in the evaporation of some of the liquid phase and the latent heat for evaporation of the liquid is compensated by a fall in the bulk liquid temperature and its saturated vapor pressure. The gas formed in this way is known as superheat gas. By removing the cargo superheat in this way, the cargo is reduced to a lower energy state and excess vapor generation on unloading is reduced or eliminated.

Where the cargo has a fuel value, as in liquefied natural gas (LNG), it has been customary to burn the boiloff gas and superheat gas as fuel for the ships boilers. This procedure, however, is now becoming uneconomical owing to the increasing price differential between natural gas and bunker fuel, coupled with the longer distances now envisaged for LNG transport. As a result, it is now becoming increasingly desirable to install appropriate equipment on board to reliquefy the evaporated cargo and return the liquid to the cargo tanks.

Several processes have been suggested for reliquefying a part or the whole of the evaporated cargo, using methods previously employed in other installations for the liquefaction of gases.

A disadvantage of these suggested processes is that they consume a considerable amount of power which is normally supplied in the form of steam from the ship s boilers. The supply of this additional steam necessitates the installation of supplementary boiler and auxiliaries and additional fuel capacity, beyond that normally needed to propel the ship, and this leads to the need for substantial modifications in the design of the vessel itself.

A second disadvantage lies in the necessity of installing the steam driven equipment in the machinery space of the ship, which is generally very congested and in which it is difficult to fit additional machines without further modifications and rearrangements of the ships equipment.

Further disadvantages of these hitherto suggested processes include an increased demand for cooling water for condensing the steam, and for other utilities such as electricity.

Yet another disadvantage of previously proposed systems is that they achieve the reliquefaction of only a small fraction of the boil-off gas.

SUMMARY OF THE INVENTION The present invention provides a method of reliquefying boil-off gas which requires neither space for equipment in the machinery area of the ship nor the utilization of steam from the ships boilers and has a very much reduced cooling water consumpton. Moreover, in one embodiment of the invention, total reliquefaction can be achieved while in another, which is applicable where the boil-off gas has a fuel value, the equipment can be substantially self-contained with a part of the boil-off gas providing the total fuel requirement for the prime mover. Even in this latter alternative, up to reliquefaction can be achieved.

In accordance with the present invention, there is provided a method of reliquefying gas resulting from evaporation of a ships cargo of liquefied gas, in which method the cold for condensing the gas is provided by a compression refrigerator including a first refrigerant loop in which a first stream of compressed refrigerant is cooled and work expanded in expansion turbine means to provide a first source of cold and the low pressure stream so obtained is subsequently recycled for recompression, and a second refrigerant loop in which a second stream of compressed refrigerant is cooled and condensed by heat exchange with the work expanded refrigerant stream and thereafter expanded without the performance of external work to cause partial vapourization thereof and to provide a cooling stream which is at the required low temperature to reliquify the gas by heat exchange with evaporation of said cooling stream, and thereafter the low pressure stream so obtained is recycled for recompression, and in which a part of the power required for compressing the refrigerant is provided by said expansion turbine means and the remainder of the power is provided by gas turbine means fueled by a gaseous or liquid hydrocarbon fuel.

It is to be understood that each of the streams of compressed refrigerant and each of the streams of low pressure refrigerant can comprise a single flow or a plurality of flows in separate conduits and that where a 3 plurality of flows are used, they may be at the same or different pressures.

An added advantage of the method of the present invention is that the hot exhaust gases from the gas turbine which carry a stoichiometric excess of oxygen can be employed as, or as part of, the combustion air to the ships propulsion engine. Alternatively, they can be used to pre-heat this combustion air and thereby increase the quantity of steam raised per unit consumption of fuel by the boilers of the propulsion engine.

In a compression refrigerator, as the term is used in this specification, refrigerant vapor is compressed, expanded with or without the performance of external work to effect cooling thereof and to provide the refrigeration, and thereafter recycled for recompression.

In accordance with one means of carrying out the invention, an open refrigeration circuit can be used with the gas resulting from the evaporation of the cargo providing at least the second stream of refrigerant. The reliquefied gas is then recovered from condensed refrigerant.

While the possibility of recovering the reliquefied gas or a part of it from condensed refrigerant in the first refrigerant loop is not excluded, in general this reliquefied gas will be recovered from condensed refrigerant in the second refrigerant loop. Where the boil-off gas is a mixture of gases (e.g. methane and nitrogen in the case of boil-off gas from liquefied natural gas), it is preferred to recover the reliquefied gas from the unvapourized portion of the refrigerant in the second refrigerant loop after expansion of the refrigerant since the composition of the condensed refrigerant stream prior to expansion will include a higher concentration of the more volatile material than is present in the cargo tanks. In any event, however, it is notgenerally convenient to recover the reliquefied gas before expansion of the refrigerant since the recovered liquefied gas would be at a much higher pressure than the liquid cargo.

The condensed refrigerant that is to be returned to the cargo tanks may be separated from the expanded refrigerant stream by any suitable means such as a vapor/liquid separator. The amount of condensed refrigerant that is returned to the cargo tank will be made equal to the amount of evaporated material entering the system from the cargo tank.

In accordance with an alternative means of carrying out the invention, a closed refrigeration circuit may be used with, for example, nitrogen or other suitable condensible gas as the refrigerant. In this arrangement, the reliquefaction of the gas evaporated from the cargo is effected by an indirect heat exchange with evaporating expanded refrigerant ,in the second refrigerant loop.

In both alternatives, low pressure refrigerant in the first refrigerant loop may be combined with low pressure refrigerant in the second refrigerant loop to form a combined low pressure refrigerant stream for recompression. The compressed refrigerant stream of the second loop may then conveniently be separated from the combined refrigerant after the combined refrigerant has been recompressed and cooled to the temperature at which the compressed refrigerant in the first refrigerant loop is work expanded. Also, the cooling of the combined compressed refrigerant may conveniently be effected by indirect heat exchange with coolant provided by' the combined low pressure refrigerant stream. Such arrangements reduce the number of separate conduits required but the two refrigerant loops may, if desired, be kept entirely separate.

Where the low pressure refrigerant streams in the two loops are at different pressures or where the low pressure refrigerant in one or both of the loops is formed of a plurality of separate flows and these flows are at different pressures, it will be necessary to equalize the pressures prior to combining the refrigerant streams of the two loops. This may conveniently be achieved by using a multistage compressor for recompressing the low pressure refrigerant and feeding the various low pressure refrigerant flows to different stages of the compressor.

In both alternatives, the cold for condensing the compressed refrigerant in the second refrigerant loop may be provided by work expanded refrigerant in the first loop and expanded refrigerant in the second loop.

Where an open refrigeration circuit is used in which the gas evaporating from the cargo provides the refrigerant and the reliquefied gas is recovered from condensed refrigerant in the second refrigerant loop, the indirect heat exchange between the expanded refrigerant in the second loop after removal of that part of the condensed portion which is'to be returned as reliquefied gas to the cargo tank, and the condensed refrigerant in the same refrigerant loop prior to expansion of said condensed refrigerant is arranged to' sub-cool the condensed refrigerant to a degree such that the subsequent Joule-Thomson expansion of the cooled condensed refrigerant will provide liquefied gas at substantially the pressure of the liquified gas cargo and at a temperature which is substantially the bubble point of the vapor above the cargo in the tank. The liquified gas is then separated from the expanded stream. The remainder of the expanded refrigerant stream in the second loop may be combined with work-expanded refrigerant in the first refrigerant loop and the combined stream used as coolant in another heat exchange step in indirect heat exchange relationship with the compressed refrigerant stream in the second refrigerant loop upstream of said one heat exchange step with reference to the direction of flow of the compressed refrigerant stream, whereby to condense the compressed refrigerant in that loop.

The same arrangement may also be employed with the closed circuit refrigeration embodiment. However, it has been found with this embodiment that satisfactory results can be obtained without the step of effecting heat exchange between the expanded refrigerant in the second loop and the compressed refrigerant prior to combining the expanded refrigerant with the work expanded refrigerant of the first loop.

In both embodiments, the refrigerant streams of the two loops may form a single .flow for recompression and the compressed single flow may then be cooled by indirect heat exchange with coolant provided by the combined low pressure stream and thereafter divided into two streams, the first being work-expanded in the first loop and the second being further cooled by indirect heat exchange with coolant provided by the combined low pressure stream, as described above, in order to condense it.

As the gas evaporated from the cargo is at a very low temperature only slightly above the boiling point of the gas at the pressure above the liquid in the cargo tank, its sensible cold may advantageously also form a part of the coolant for cooling of the compressed refrigerant in each of the refrigerant loops. In the case of the open refrigerant circuit alternative, the gas may conveniently be combined with low pressure refrigerant. Thus, for

example, in the preferred embodiment where the low pressure refrigerant streams of the two loops are combined prior to heat exchange with the compressed refrigerant, the gas may be fed into the combined stream, also prior to the heat exchange.

Further according to the present invention, there is provided apparatus suitable for use with the open circuit refrigeration embodiment of the invention, said apparatus including: refrigerant compressing means comprising first and second compressor means arranged for passage of gaseous refrigerant therethrough in series,

expansion turbine means operatively connected to one of said first and second compressor means to drive same,

hydrocarbon fueled gas turbine means operatively connected to the other of said first and second compressor means to drive same,

expansion valve means, and

first, second and third heat exchange means,

and further including:

first conduit means adapted to pass a first stream of compressed refrigerant from said first and second compression means through said first heat exchange means to cool it and then through said turbine expansion means to work expand it to provide a first low pressure refrigerant stream,

second conduit means adapted to pass a second stream of compressed refrigerant through said first, second and third heat exchange means in series to cool and condense the stream and then through said expansion valve means whereby to cause partial vapourization of the condensed stream,

means for recovering at least a part of the unvapourized portion of the partially vapourized stream,

third conduit means adapted to pass a second low pressure refrigerant stream comprising the unrecovered part of the partially vapourized stream in countercurrent indirect heat exchange relationship with said second stream of compressed refrigerant in said third heat exchange means,

fourth conduit means for passing both said second low pressure refrigerant stream leaving said third heat exchange means and said first low pressure refrigerant stream in counter-current indirect heat exchange relationship with said second stream of compressed refrigerant in said second heat exchange means and thereaf ter in counter-current indirect heat exchange relationship with said second and first streams of compressed refrigerant in said first heat exchange means and thereafter returning said first and second low pressure refrigerant streams to said refrigerant compressing means,

fifth conduit means adapted to feed gas evaporated from the liquefied gas cargo into admixture with low pressure refrigerant, and

sixth conduit means for passing the recovered part of the unvapourized portion of the partially vaporized stream for return to the liquefied gas cargo.

The invention also provides apparatus suitable for use with the closed circuit refrigeration embodiment of the present invention, said apparatus comprising a compression refrigerator including:

refrigerant compressing means comprising first and second compressor means arranged for passage of gaseous refrigerant therethro ugh in series,

expansion turbine means operatively connected to one of said first and second compressor means to drive same,

hydrocarbon fueled gas turbine means operatively connected to the other of said first and second compressor means to drive same,

expansion valve means, and

first, second and third heat exchange means,

and further including:

first conduit means adapted to pass a first stream of compressed refrigerant from said first and second compression means through said first heat exchange means to cool it and then through said turbine expansion means to work expand it to provide a first low pressure refrigerant stream,

second conduit means adapted to pass a second stream of compressed refrigerant through said first and second heat exchange means in series to condense the stream and then through said expansion valve means whereby to cause partial vapourization of said condensed stream to form a second low pressure refrigerant stream,

third conduit means adapted to pass said second low pressure refrigerant stream through said third heat exchange means,

fourth conduit means adapted to pass gas evaporated from the liquefied gas cargo through said third heat exchange means in indirect heat exchange relationship with said second low pressure refrigerant stream and to pass liquid so formed for return to said liquefied gas cargo,

fifth conduit means adapted to pass both said second low pressure refrigerant stream leaving said third heat exchange means and said first low pressure refrigerant stream in counter-current indirect heat exchange relationship with said second stream of compressed refrigerant in said second heat exchange means and thereafter in counter-current indirect heat exchange relationship with said second and first streams of compressed refrigerant in said first heat exchange means and thereafter returning said first and second low pressure refrigerant stream to said refigerant compressing means.

The apparatus of both embodiments lend themselves to being assembled in self-contained modules for easy installation in the ship. This avoids another disadvantage of installing reliquefaction process equipment on board, which is that shipyards are not generally sufficiently familiar with this type of equipment and this can lead to poor installation work, higher costs and lack of proper testing facilities. Thus, for example, three modules can be formed, one containing the gas turbine and the compressor which it drives, a second containing the expansion turbine and the compressor which it drives, and the third comprising a cold box including the necessary heat exchangers, the expansion valve or valves and ancillary equipment. Each module can be readily assembled and tested separately by a specialist fabricator using personnel familiar with the equipment, and away from the ship, and thereafter installed on the ship when only a minimum inolvement in assembly and testing work will be required. This results in better workmanship, lower costs and more thorough testing.

The invention is particularly suitable for the reliquefaction of gas having a fuel value, the most common example being the boil-off gas and superheat gas from liquefied natural gas (LNG). In accordance with one preferred embodiment, a portion of the gas, which can be as little as 20% of the total, whereby up to is available for reliquefaction, may be used to fuel the gas turbine means which is suitably adapted for that purpose.

Where a portion of the gas is used to fuel the gas turbine, it is desirable to warm this portion to near or above ambient temperature prior to feeding it to the gas turbine. This is preferably achieved by passing it in indirect heat exchange with compressed gaseous refrigerant, which also assists in cooling the latter.

Where closed circuit refrigeration is used, the gas may be divided into two streams, one of which is heatexchanged with compressed refrigerant in order to warm it prior to supplying it as fuel to the gas turbine and the other of which is heat exchanged with expanded refrigerant in the second loop in order to reliquefy it for return to the cargo.

Where the gas not only is used to fuel the gas turbine but also supplies the refrigerant; i.e. where an open refrigeration circuit is employed, the gas stream may conveniently be combined with low pressure gaseous refrigerant which is thereafter passed in indirect heat exchange relationship with the compressed refrigerant prior to recompression. A portion of this combined stream is thereafter separated out for feeding to the ga turbine as fuel.

Where the gas evaporated from the cargo tank is at an insufficiently high pressure for feeding to the gas turbine, it can be compressed by a pump provided for that purpose and the pump may be driven by the gas turbine. Where the gas is also being used as the refrigerant, however, the need for such a pump may be avoided by bleeding the fuel stream from the refrigerant after at least partial compression of the refrigerant in the compressors. In one preferred arrangement where two compressors are used to compress the refrigerant, one being driven by the gas turbine, a portion of the refrigerant recovered from the first of the two compressors may be passed as fuel to the gas turbine which, for convenience, may be arranged to drive this first compressor. The remainder of the stream recovered from the first compressor may then be passed to the second compressor for final compression and subsequent use in the two refrigeration loops. The second compressor will in this case be driven by the expansion turbine.

It will be appreciated that in the case where the gas from the cargo provides both fuel for the gas turbine and refrigerant, the amount of reliquefied gas that is removed from the condensed refrigerant will be arranged to be equivalent to the amount of gas evaporating from the cargo less the quantity consumed as fuel.

Improved control of the process of the present invention may be achieved by dividing the stream of condensed refrigerant in the second loop into two or more flows of refrigerant which are expanded to different pressures.

The stream of condensed refrigerant may suitably be divided into two flows by use eg of a gas/liquid separator subsequent to a preliminary expansion of the condensed refrigerant in which partial revapourization occurs. Thus, a part of the liquid recovered from the separator may provide one of said flows and may be further expanded to provide the lower pressure flow, and the remainder of the liquid from the separator together with the gas from the separator may form the other of said flows. This combined gas/liquid flow may also be further expanded, either before or after the'combination of the gas and the liquid, but to not as low a pressure as the first-mentioned flow.

BRIEF DESCRIPTION OF THE DRAWINGS The various features of the invention are now illustrated in more detail with reference to preferred em bodiments and with the aid of the accompanying drawings in which:

FIG. 1 is-a generalized schematic flow sheet illustrating an embodiment of the open refrigeration circuit alternative of the present invention wherein gas evaporated from the cargo is utilized as refrigerant with total reliquefaction;

FIG. 2 is a similarly generalized schematic flow sheet illustrating another embodiment of the open refrigeration circuit alternative of the present invention in which gas evaporated from the cargo is utilized both as refrigerant and as fuel for the gas turbine;

FIG. 3 is a generalized schematic flow sheet illustrating an embodiment of the closed refrigeration circuit alternative of the present invention, a portion of the gas evaporated from the cargo being used as fuel for the gas turbine; and

FIG. 4 is a detailed flow sheet illustrating a refinement of the embodiment illustrated in FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, reference numeral 2 is the cargo tank of a tanker ship containing liquefied gas 4, 6 is a gas turbine fueled by a liquid hydrocarbon fuel such as fuel oil, 8 is a two-stage compressor, 10 is another compressor, 12 is an expansion turbine, 14 is an expansion valve, 16 is a gas/liquid separator and 18, 20 and 22 are heat exchangers.

The cold gas evaporating from the cargo in tank 2, and which is at a temperature of about 158C, leaving the tank by pipeline 30 in which it warms up to about 138C, is combined with low pressure refrigerant which is returning to the compressors in lines 32 and 42, and the combined cold low pressure stream passes in pipeline 34 through heat exchanger 20 where it forms the coolant stream hereinafter referred to as the first coolant stream and then heat exchanger 18 where it forms the coolant stream referred to hereinafter as the second coolant stream. During its passage through these heat exchangers, it is warmed to about ambient temperature. It is then fed via pipeline 36 to compressor 8 where it is compressed in two stages to about 200 psia. The gas from compressor 8 is then fed to compressor 10 where it is further compressed to about 260 psia, and then passed via pipeline 38 back through heat exchanger 18 where it is cooled by the aforementioned second coolant stream and divided ito two streams of compressed refrigerant in pipelines 40 and 44 referred to hereinafter as first and second streams of compressed refrigerant.

In a first refrigerant loop, said first stream is passed via pipeline 40 to expansion turbine 12 where it is expanded to about atmospheric pressure. This expansion causes the temperature of the stream to fall below the bubble point of the stream at the pre-expansion pressure of 260 psia. The cold low pressure stream so obtained forms a first low pressure refrigerant stream which is then passed through pipeline 42 to join up with the incoming gas in pipeline 30 and a returning low pressure refrigerant stream described in more detail below in pipeline 32, to form the aforementioned first coolant stream.

In a second refrigerant loop, the second stream of compressed refrigerant is passed via pipeline 44 through heat exchanger 20 where it is cooled further and completely condensed by indirect counter-current heat exchange with the aforementioned first coolant stream in pipeline 34, the consitution of which has already been described. The condensed stream is then passed through heat exchanger 22 in which it is subcooled. The sub-cooled liquid is then passed by pipeline 46 to expansion valve 14 where it is expanded to about 18 psia which is slightly above the pressure above the cargo, and some evaporation takes place as the stream is slightly above its bubble point at this pressure. The low pressure partly vapourized stream is then passed to gas/liquid separator 16. A part of the liquid which is recovered, equivalent to the gas leaving the tank in pipe 30, is returned to the tank 2 by pipeline 48. The remainder is passed through pipeline 50 to join the gas leaving the separator in pipeline 52.

Some further evaporation will occur in pipeline 48 because of the pressure drop from about 18 psia to the pressure of about psia pertaining in the tank and this will add slightly to the total gas in pipeline 30.

The low pressure gas/liquid mixture in pipeline 52, formiing a second low pressure refrigerant stream, is passed back through exchanger 22 where it is evaporated and subcools the condensed liquid in line 46 to the degree required for the sub-cooled liquid to provide, after expansion, a vapor portion and a liquid portion, said liquid portion being at a temperature which is equal to the bubble point of the vapor above the liquid in the tank 2 and at substantially tank pressure. Thence, the evaporated low pressure stream passes through line 32 to be combined with the incoming gas in line 30 and the exhaust gas from the turbine 12 in line 42 for return to the compressors as described above. The difference between the pressure of the gas in pipeline 52 and that of the incoming gas in line 30 is accounted for by an appropriate pressure drop through heat exchanger 22.

It will be appreciated that the above scheme is capable of dealing with the variable amounts of nitrogen that will occur in LNG. Nitrogen is, of course, more volatile than methane, which is the principal constituent of LNG. Thus, the gas in line 30 will have a higher nitrogen content than the LNG 4 in the tank 2. The liquid returned to the tank in pipeline 48 has the same nitrogen content as the gas in line 30 and thus the liquid in separator 16 W11 have a higher nitrogen content than the liquid in the tank because of the evaporation that occurs in pipeline 48. Moreover, the liquid/vapor mixture in pipeline 52 will contain more nitrogen than the mixture arriving at expansion valve 14. Thisenables the mixture in separator 16 to evaporate at a sufficiently low temperature to sub-cool the condensed stream in heat exchanger 22. The arrangement is thus able to handle LNG with a wide variety of nitrogen contents.

Moreover, the arrangement can be substantially independent of the ships machinery and services since the two-stage compressor 8 is driven by gas turbine 6 which can be fueled by any suitable liquid or gaseous hydrocarbon fuel, and compressor 10 is driven by expansion turbine 12. The equipment can also be assembled for minimum installation work on board the tanker by mounting the gas turbine 6 and two-stage compressor 8 with the necessary ancillary equipment on one skid, compressor 10 and expansion turbine 12 with their ancillary equipment on another skid, and the 10 three heat exchangers, expansion valve 14 and separator 16 in a cold box on a third skid. The equipment can than be coupled together when the skids have been installed in the ship.

It will also be appreciated that the heat exchange arrangements can be varied without departing from the invention. Thus, for example, in one alternative arrangement, the incoming gas from cargo tank 4 can be combined with the low pressure refrigerant stream in line 32 and then injected into the exhaust gas from the turbine 12 between heat exchangers 20 and 18. With this arrangement, however, the turbine 12 would have to process more gas in order to satisfy the cold requirements of heat exchanger 20.

Referring now to FIG. 2, the same items of equipment as those in the arrangement of FIG. 1 carry the same reference numerals but in this case the gas turbine identified by reference numeral 6 is adapted to be fueled by boil-off gas and the compressor 8 is a single stage compressor. v

Also, heat exchangers l8 and 20 are replaced by a single heat exchanger 100.

As in the arrangement of FIG. 1, an open refrigeration circuit is used with the boil-off gas providing the refrigerant, as well as but in the arrangement of FIG. 2, the boil-off gas also provides the fuel to the gas turbine 6.

Referring to FIG. 2, all the cold gas evaporated from the cargo leaves the cargo tank at about 158C through pipeline 102 and, as in the arrangement in FIG. 1, is combined which low pressure refrigerant in pipelines 116 and 128. The cold combined stream at a temperature of about 138C. is introduced into heat exchanger through pipeline 104 and is warmed to ambient temperature (about 15C) therein by indirect countercurrent heat exchange with compressed hot refrigerant. Leaving the heat exchanger 100 by pipeline 106, the gas is fed to a first compressor 8 which is driven by gas turbine 6 and in which the gas is compressed to an intermediate pressure. A small proportion of it (e.g. about 20%) is then fed by pipeline 108 as fuel to the gas turbine 6 while the major proportion is passed through pipeline 110 for further compression in a second compressor 10 which is driven by expansion turbine 12. The high pressure gas at 260 psia is then passed by pipeline 112 to the first section of heat exchanger 100 where it is cooled by counter-current indirect heat exchange with coolant comprising the cold combined gas stream in pipeline 104/106. This coolant stream in the first section of heat exchanger 100 is equivalent to the coolant stream referred to as the second coolant stream in the embodiment illustrated in FIG. 1. A part of the high pressure gas forms the second compressed refrigerant stream and passes through the whole of the heat exchanger 100 while the remainder forms the first compressed refrigerant stream and is withdrawn in pipeline 114 at an intermediate point along the exchanger and expanded in expansion turbine 12 to about 15 psia to provide the refrigeration to condense the second compressed refrigerant stream. The cold expanded gas from the expansion turbine, forming the first low pressure refrigerant stream, is then passed through pipeline 116 to be combined with the incoming gas in pipeline 102 and returned via pipeline 104 through heat ex changer 100 to provide the coolant and then to the compressors. The part of the high pressure gas which passes through the whole length of the heat exchanger 100 is cooled and condensed in the last section thereof by indirect heat exchange with the cold combined gas stream in pipeline 104/ 106 which in effect provides in this last section of the heat exchanger the coolant stream equivalent to that referred to as the first coolant stream in the embodiment illustrated in FIG. 1. The condensed stream then passes via pipeline 118 to heat exchanger 22 where it is subcooled and is thereafter expanded in expansion valve 14 from which it emerges as a mixture of liquid and vapor in pipeline 120 in the manner described with reference to FIG. 1. An amount of liquid equivalent to the amount of gas fed to the plant less the portion burned in the gas turbine is then withdrawn through pipeline 122 and returned to the LNG cargo tank 2. The rest of the liquid (via pipeline 124) together with the gas in pipeline 126 forms the second low pressure refrigerant stream and is returned through heat exchanger 22 in pipeline 128 to sub-cool the condensed refrigerant stream in pipeline 118. It is then fed into pipeline 104 to join the incoming gas and the exhaust gas from expansion turbine 12 for return through heat exchanger 100.

As in the arrangement of FIG. 1, the equipment can suitably be mounted on three skids for installation in the ship.

In the arrangement illustrated in FIG. 3, wherein the items of equipment which are the same as those used in FIG. 2 are identified by the same reference numerals, a closed refrigeration circuit is used but part of the boiloff gas is used to fuel the gas turbine 6.

In the method of the variant of FIG. 3, cold gas evaporated from the cargo enters the plant in pipeline 202 and is divided into two streams in pipelines 204 and 206 respectively. The larger stream in pipeline 206 is completely reliquefied in heat exchanger 22 by indirect counter-current heat exchange with evaporating low pressure cold refrigerant and returned to the cargo tank in pipeline 208. The smaller stream in pipeline 204 is passed into heat exchanger 100 where it is warmed to ambient temperature by indirect heat exchange with compressed refrigerant in pipeline 210 and fed as fuel to the gas turbine 6 where it is mixed with air entering through pipeline 212.

In the refrigerant cycle, nitrogen at a high pressure, about 650 psia,- is passed into heat exchanger 100 in pipeline 210. As in the embodiment illustrated in FIG. 2, in the first section of the heat exchanger it is cooled by indirect heat exchange with coolant comprising the cold gas in pipeline 204 and the returning expanded refrigerant in pipeline 224, this coolant corresponding to that referred to as the second coolant stream in the embodiment of FIG. 1. Part of the high pressure nitrogen comprising the second stream of compressed refrigerant proceeds through the whole length of this heat exchanger and is substantially condensed in the last section of this heat exchanger by heat exchange with coolant comprising the cold gas in pipeline 204 and returning expanded refrigerant in pipeline 224, the coolant in this last section of the heat exchanger corresponding to that referred to as the first coolant stream in the embodiment of FIG. 1. The condensed stream leaves at a temperature of 160C by pipeline 218 by which it is led to expansion valve 14 in which it is expanded to a lower pressure, about 105 psia, as a result of which a liquid/vapor mixture is formed at about 173C. The low pressure liquid/vapor mixture from the expansion valve which forms the second low pressure refrigerant stream is then passed by pipeline 220 through heat exchanger 22 where it evaporates and thereby cools and reliquefies the gas in pipeline 206. The low pressure nitrogen leaves the exchanger at a temperature of about 165C in pipeline 222. The remainder of the high pressure nitrogen, constituting the first compressed refrigerant stream, is withdrawn at a temperature of about C in pipeline 214 at an intermediate point of heat exchanger 100 and expanded to a lower pressure, about 100 psia, in expansion turbine 12 thereby producing the required refrigeration. The cold low pressure gaseous nitrogen from this turbine, which is at a temperature of about f 165C, is then passed through pipeline 216 to be combined with the low pressure nitrogen in pipeline 222 and the combined low pressure stream is passed back through heat exchanger 100, where it is warmed by heat exchange with the high pressure nitrogen in pipeline 210 to near room temperature, simultaneously cooling and condensing the nitrogen in pipeline 218.

This gas is then passed by pipeline 226 to first compressor 10 driven by expansion turbine 12 and is compressed to an intermediate pressure. Thence it is passed by pipeline 228 to the second compressor 8 driven by gas turbine 6, where it is compressed to the full pressure of 650 psia for recirculation to pipeline 210.

The various pressures and temperatures pertaining in the system are tabulated below.

TABLE 1 Pipeline Temp. Press.

0C psia 202 138 I5 204 l 38 I5 206 l 38 I5 208 171 14.7 210 20 650 214 100 645 216 165 I00 218 640 220 173 I05 224 I00 226 I5 95 -by a two-stage compressor 302, expansion valve 14 is replaced by expansion valves 304, 306, 308 and 310 and gas/liquidseparator 312, the heat exchanger 100 is replaced by heat exchangers 314 and 316 and heat exchanger 22 is replaced by heat exchanger 318.

As shown in this Figure, the cold gas evaporated from the cargo enters at a temperature of about l38C and a pressure of about 15 psia through pipeline 320. A major portion of this gas (about 80%) is passed via pipeline 322 through heat exchanger 318, which corresponds to heat exchanger 22 of FIG. 3, in which it is completely liquefied. The liquid collects in receiver 324 and is returned to the LNG cargo tanks, by means of pump 326.

The remainder of the incoming gas (about 20%) passes in pipeline 328 through heat exchangers 314 and 316 where it forms part of the coolant for each of these heat exchangers and is warmed to near ambient temperature. These exchangers together correspond to 13 heat exchanger 100 of FIG. 3. It is then fed through olower pump 330 where it is pressurized to about 50 psia to be burned in the gas turbine 6 with air entering through pipe 332.

The gas turbine 6 drives centrifugal compressor 8, which compressed a stream of nitrogen from an intermediate pressure, about 100 psia, and which enters through pipe 334, to an elevated pressure of about 650 psia. The compressed nitrogen leaves the compressor through pipe 336 and, after passing through cooler 338 which cools it to about ambient temperature, is passed via pipeline 340 to exchanger 316 where it is cooled to about 100C. Thereafter, a first part proceeds through exchanger 314 where it is cooled further to about 165C and condensed and is thereafter expanded to an intermediate pressure of about 105 psia in valve 304 in the course of which a part is revapourized and the stream is further cooled to about 173C. The liquid/gas mixture then enters gas/liquid separator 312. The liquid is withdrawn from the separator through pipeline 342 and then divided into two streams. The first stream is further expanded to 100 psia and cooled in expansion valve 308 and then combined with vapor which is withdrawn from the separator through pipeline 344 and similarly expanded in expansion valve 306. The combined stream is then passed through exchanger 318, where the liquid is evaporated. The second stream of liquid from the separator is expanded to about 20 psia in valve 310 and also passed through exchanger 318 where it is also evaporated. Evaporation of both streams causes liquefaction of the gas in pipeline 322.

The two nitrogen streams leaving heat exchanger 318, which are at pressures of 100 psia and 20 psia, are then passed in pipelines 348 and 346 respectively to exchangers 314 and 316 where they are warmed to ambient temperature by the compressed refrigerant in pipeline 340.

The remainder of the high pressure gas leaving exchanger 316 is diverted through pipeline 350, and is expanded in turboexpander 12 to the same pressure as the nitrogen stream in pipeline 348, i.e. about 100 psia. It is then returned by line 352 to join the stream of pipeline 348, this being the higher pressure stream of the two low pressure nitrogen streams leaving heat exchanger 318.

Turbo-expander 12 drives a multistage compressor 302. The lower pressure nitrogen at about 20 psia from pipeline 346 enters the first stage 302a of compressor 302 through line 354 and then passes through aftercooler 355. Nitrogen at about 95 psia from pipeline 348 enters an intermediate stage 302b of compressor 302 through line 356. Compressor 302 then delivers the nitrogen at about 100 psia through line 334 to the centrifugal compressor 8 for recompression to 650 psia and recirculation.

The various pressures and temperatures pertaining in the system are tabulated below.

TABLE l-continued Pipeline Temp. Press.

0C psia 346 165 20 348 165 100 350 "I00 645 352 l65 I00 354 l5 I5 356 I5 95 The plant for carrying out the method of the invention can suitably be pre-assembled on three skids. One caries the low temperature heat exchangers and ancillaries such as liquid/vapor separators, fully assembled with interconnecting pipework in an insulated casing. The second carries the expansion turbine and the compressor which it drives, and the third carries the gas turbine and the compressor which it drives.

All three skids can be placed in position on a tanker in a fully assembled condition requiring only a minimum of site work for installing the pipework between the individual modules and connecting the services.

None of the skids need be placed in the machinery space of the vessel and in particular the gas turbine driven compressor can be installed on deck.

The foregoing description is directed primarily to the reliquefaction of boil-off gas which is evaporated from a liquid gas cargo which will be slightly above atmospheric pressure. In order to provide a greater pressure drop across the heat exchangers of the reliquefaction plant is may be desirable in some circumstances to increase the pressure of the boil-off gas prior to feeding it to the reliquefaction plant. Raising the pressure of the boil-off gas will also have the effect of reducing the load on the reliquefaction plant since the reliquefaction temperature of the boil-off gas will also be raised. It will also reduce or eliminate any further compression of the gas which may otherwise be required, when it is intended to utilize a portion of it to fuel another gas turbine.

Conveniently, this compression of the boil-off gas can be effected using the boil-off gas compressor that is a usual part of a liquefied gas tanker ships equipment. The employment of the compressor for this purpose provides an additional element of safety in that it ensures its immediate availability for partial reliquefaction of the boil-off gas in the event that use of the reliquefaction plant has to be discontinued.

I claim:

1. A method of reliquefying gas resulting from evaporation of a ships cargo of liquefied gas, said method comprising the steps of h i. providing a first stream of compressed gaseous refrigerant;

ii. cooling said stream and work expanding it to provide a first low pressure refrigerant stream which is at a temperature which is below the bubble point of said compressed gaseous refrigerant;

iii. condensing a second stream of compressed gaseous refrigerant by indirect heat exchange with first coolant means provided at least in part by said first low pressure refrigerant stream;

iv. cooling the fluid to be returned as a liquid to the ships cargo so that it will attain a temperature which is substantially equal to the bubble point of the vapor above said cargo when the pressure of said gas is substantially equal to the pressure of said cargo, said cooling being effected by indirect heat exchange between said fluid and at least one second low pressure refrigerant stream with evaporation of said stream, said at least one second low pressure refrigerant stream being provided by effecting Joule-Thomson expansion of said condensed second stream to effect partial vaporization thereof; and

v. recovering from said cooled gas a condensate which is at substantially the pressure of the ships cargo for return to said liquefied gas cargo;

said cooling of said first stream of compressed gaseous refrigerant being effected by indirect heat exchange with second coolant means provided at least in part by at least one of said first and second low pressure refrigerant streams, and said first and second low pressure refrigerant streams being thereafter recompressed to provide said first and second stream of compressed refrigerant by compressor means driven by gas turbine means fueled by a fluid hydrocarbon fuel and by said expansion turbine means.

2. A method as claimed in claim 1 in which the combustion air to the ships propulsion engine includes hot exhaust gas from the gas turbine.

3. A method as claimed in claim 1 in which combustion aiar to the ships propulsion engine is pre-heated by hot exhaust gas from the gas turbine.

4. A method as claimed in claim 1 in which the said first coolant means includes said first and second low pressure refrigerant streams.

5. A method as claimed in claim 1 in which the said first low pressure refrigerant stream is combined with said second low pressure refrigerant stream to form a combined low pressure refrigerant stream which provides at least part of each of said first and second coolant means, said combined stream being thereafter passed to said compressor means for recompression and said first and second streams of compressed refrigerant being derived from the recompressed combined stream after said recompressed combined stream has been cooled by said heat exchange with said second coolant means.

6. A method as claimed in claim 1 in which said first and second coolant means each further include a stream of gas resulting from evaporation of the cargo.

7. A method as claimed in claim 1 in which at least said second stream of compressed refrigerant is derived from said gas evaporated from the cargo, and wherein step (iv) is effected by indirect heat exchange between said condensed second stream of compressed refrigerant and said second low pressure refrigerant stream prior to effecting Joule-Thomson expansion of said condensed second stream, said condensate being recovered from said condensed second stream after said Joule-Thomson expansion, and the remainder of said expanded second stream providing said second low pressure refrigerant stream.

8. A method as claimed in claim 7 in which the said first low pressure refrigerant stream is combined with said second low pressure refrigerant stream to form a combined low pressure refrigerant stream which provides at least part of each of said first and second coolant means, said combined stream being thereafter passed to said compressor means for recompression and said first and second streams of compressed refrigerant being derived from the recompressed combined stream after said recompressed combined stream has- 16 been cooled by said heat exchange with said second coolant means.

9. A method as claimed in claim 8 in which said gas evaporated from the cargo is injected into said combined low pressure refrigerant stream and is contained in said first and second coolant means.

10. A method as claimed in claim 7 in which said gas evaporated from the cargo has a fuel value, a portion of said gas is used to fuel said gas turbine and said condensate is recovered from said condensed second stream and returned to the cargo at a rate which is equal to the rate of evaporation of gas from the cargo less the rate of consumption of the evaporated cargo as fuel.

11. A method as claimed in claim 10 in which said portion is derived from at'least partially recompressed refrigerant.

12. A method as claimed in claim 1 in which said second stream of low pressure refrigerant consists of said expanded condensed second stream of compressed refrigerant and said gas evaporated from the cargo is reliquefied by indirect heat exchange with said second low pressure refrigerant stream.

13. A method as claimed in claim 12 in which the liquefied gas and the refrigerant is nitrogen.

14. A method as claimed in claim 12 in which said first low pressure refrigerant stream is combined with said second low pressure refrigerant stream to form a combined low pressure refrigerant stream which provides at least part of each of said first and second coolant means, said combined stream being thereafter passed to said compressor means for recompression and said first and second streams of compressed refrigerant being derived from the recompressed combined stream after said recompressed combined stream has been cooled by said heat exchange with said second coolant means.

15. A method as claimed in claim 12 in which said Joule-Thomson expansion is effected in at least two steps whereby said second stream of low pressure refrigerant is provided in the form of a plurality of separate flows at different pressures.

16. A method as claimed in claim 15 in which at least one of said flows is at substantially the same pressure as said first low pressure refrigerant stream and is combined therewith to form a combined low pressure refrigerant stream which provides part of each of said first and second coolant means, the remainder of said second low pressure refrigerant stream is at lower pressure and provides another part of each of said first and second coolant means, said remainder being thereafter compressed to substantially the pressure of said combined low pressure refrigerant stream and then combined therewith and the stream so produced being recompressed to form a stream from which said first and second streams of compressed refrigerant are derived after said recompressed stream has been cooled by said heat exchange with said second coolant means.

17. A method as claimed in claim 12 in which said first and second coolant means each further include a stream of gas resulting from evaporation of the cargo, said gas stream being separate from the gas to be reliquefied.

18. A method as claimed in claim 12 in which the gas resulting from the evaporation of the ships cargo has a fuel value and a portion of said gas is-used to fuel the gas turbine, said portion being separate from the gas to be reliquefied.

17 19. A method as claimed in claim 18 in which said portion provides a part of each of asidfirst and second coolant means before it is supplied to said gas turbine.

20. A method as claimed in claim 7 in which the liquefied gas is liquefied natural gas.

21. Ship-board apparatus for the reliquefaction of gas resulting from evaporation of a ships cargo of liquefied gas, said apparatus including:

refrigerant compressing means comprising first and second compressor means arranged for passage of gaseous refrigerant therethrough in series;

expansion turbine means operatively connected to one of said first and second compressor means to drive same;

hydrocarbon fueled gas turbine means operatively connected to the other of said first and second compressor means to drive same; expansion valve means; and first, second and third heat exchange means; and further including:

first conduit means for passing a first stream of compressed refrigerant from said first and second compression means through said first heat exchange means to cool it and then through said turbine expansion means to work expand it to provide a first low pressure refrigerant stream; second conduit means for passing a second stream of compressed refrigerant through said first, second and third heat exchange means in series to cool and condense the stream and then through said expansion valve means whereby to cause partial vaporization of the condensed stream; means for recovering at least a part of the unvaporized portion of the partially vaporized stream;

third conduit means for passing a second low pressure refrigerant stream comprising the unrecovered part of the partially vaporized stream in counter-current indirect heat exchange relationship with said second stream of compressed refrigerant in said third heat exchange means; fourth conduit means for passing both said second low pressure refrigerant stream leaving said third heat exchange means and said first low pressure refrigerant stream in counter-current indirect heat exchange relationship with said second stream of compressed refrigerant in said second heat exchange means and thereafter in counter-current indirect heat exchange relationship with said second and first streams of compressed refrigerant in said first heat exchange means and thereafter returning said first and second low pressure refrigerant streams to said refrigerant compressing means;

fifth conduit means for feeding gas evaporated from the liquefied gas cargo into admixture with low pressure refrigerant; and

sixth conduit means for passing the recovered part of the unvaporized portion of the partially vaporized stream for return to the liquefied gas cargo.

22. Apparatus as claimed in claim 21 in which said gas turbine is adapted to be fueled by gas evaporated from the liquid gas cargo, said apparatus also including means for feeding a portion of refrigerant recovered from said first compressor means to said gas turbine means as fuel.

23. Apparatus as claimed in claim 21 in which the expans on valve means is adapted to provide said second low pressure refrigerant stream as a plurality of flows at different pressures.

'18 24. Apparatus as claimed in claim 21 arranged in three separable modules, the first of which contains one compressor means and the gas turbine means operatively connected to said compressor means to drive same, the second of which includes the second compressor means and the expansion turbine means operatively connected to said second compressor means to drive same, and the third of which includes a cold box containing said heat exchange means and said expansion valve means. i

25. Ship-board apparatus for the reliquefaction of gas resulting from evaporation of a ships cargo of liquefied gas, said apparatus comprising:

refrigerant compressing means comprising first and second compressor means arranged for passage of gaseous refrigerant therethrough in series;

expansion turbine means operatively connected to one of said first and second compressor means to drive same;

hydrocarbon fuel gas turbine means operatively connected to the other of said first and second compressor means to drive same;

expansion valve means; and

first, second and third heat exchange means; and further including:

first conduit means for passing a first stream of compressed refrigerant from said first and second compression means through said first heat exchange means to cool it and then through said turbine expansion means to work expand it to provide a first low pressure refrigerant stream;

second conduit means for passing a second stream of compressed refrigerant through said first, second and third heat exchange means in series to cool and condense the partial vaporization of said condensed stream to form a second low pressure refrigerant stream;

third conduit means for passing said second low pressure refrigerant stream through said third heat exchange means;

fourth conduit means for passing gas evaporated from the liquefied gas cargo through said third heat exchange means in indirect heat exchange relationship with said second low pressure refrigerant stream and to pass liquid so formed for return to said liquefied gas cargo; and

fifth conduit means for passing both said second low pressure refrigerant stream leaving said third heat exchange means and said first low pressure refrigerant stream in counter-current heat exchange relationship with said second stream of compressed refrigerant in said second heat exchange means and thereafter in counter-current indirect heat exchange relationship with said second and first streams of compressed refrigerant in said first heat exchange means and thereafter returning said first and second low pressure refrigerant streams to said refrigerant compressing means.

26. Apparatus as claimed in claim 25 in which said gas turbine means is adapted to be fueled by gas evapo rated from the liquid gas cargo, said apparatus further including conduit means for passing a portion of the evaporated gas from the cargo in indirect counter-current heat exchange with said compressed refrigerant to warm said portion and then to said gas turbine means as fuel.

27. Apparatus as claimed in claim 25 in which the expansion valve means is adapted to provide said second pressor means and the expansion turbine means operatively connected to said second compressor means to drive same, and the third of which includes a cold box containing said heat exchange means and said expansion valve means.

UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION PATENT NO. 3,

DATED November 18, 1975 INVENTOR(S) James Kevin JONES It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 10, line 25, delete "as well as" Column 15, line 27, claim 3, delete "aiar" and insert therefor --air-- Signed and Eaealcd ttu's sixteenth D ay Of March I 9 76 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Parenrs and Trademarks 

2. A method as claimed in claim 1 in which the combustion air to the ship''s propulsion engine includes hot exhaust gas from the gas turbine.
 3. A method as claimed in claim 1 in which combustion aiar to the ship''s propulsion engine is pre-heated by hot exhaust gas from the gas turbine.
 4. A method as claimed in claim 1 in which the said first coolant means includes said first and second low pressure refrigerant streams.
 5. A method as claimed in claim 1 in which the said first low pressure refrigerant stream is combined with said second low pressure refrigerant stream to form a combined low pressure refrigerant stream which provides at least part of each of said first and second coolant means, said combined stream being thereafter passed to said compressor means for recompression and said first and second streams of compressed refrigerant being derived from the recompressed combined stream after said recompressed combined stream has been cooled by said heat exchange with said second coolant means.
 6. A method as claimed in claim 1 in which said first and second coolant means each further include a stream of gas resulting from evaporation of the cargo.
 7. A method as claimed in claim 1 in which at least said second stream of compressed refrigerant is derived from said gas evaporated from the cargo, and wherein step (iv) is effected by indirect heat exchange between said condensed second stream of compressed refrigerant and said second low pressure refrigerant stream prior to effecting Joule-Thomson expansion of said condensed second stream, said condensate being recovered from said condensed second stream after said Joule-Thomson expansion, and the remainder of said expanded second stream providing said second low pressure refrigerant stream.
 8. A method as claimed in claim 7 in which the said first low pressure refrigerant stream is combined with said second low pressure refrigerant stream to form a combined low pressure refrigerant stream which provides at least part of each of said first and second coolant means, said combined stream being thereafter passed to said compressor means for recompression and said first and second streams of compressed refrigerant being derived from the recompressed combined stream after said recompressed combined stream has been cooled by said heat exchange with said second coolant means.
 9. A method as claimed in claim 8 in which said gas evaporated from the cargo is injected into said combined low pressure refrigerant stream and is contained in said first and second coolant means.
 10. A method as claimed in claim 7 in which said gas evaporated from the cargo has a fuel value, a portion of said gas is used to fuel said gas turbine and said condensate is recovered from said condensed second stream and returned to the cargo at a rate which is equal to the rate of evaporation of gas from the cargo less the rate of consumption of the evaporated cargo as fuel.
 11. A method as claimed in claim 10 in which said portion is derived from at least partially recompressed refrigerant.
 12. A method as claimed in claim 1 in which said second stream of low pressure refrigerant consists of said expanded condensed second stream of compressed refrigerant and said gas evaporated from the cargo is reliquefied by indirect heat exchange with said second low pressure refrigerant stream.
 13. A method as claimed in claim 12 in which the liquefied gas and the refrigerant is nitrogen.
 14. A method as claimed in claim 12 in which said first low pressure refrigerant stream is combined with said second low pressure refrigerant stream to form a combined low pressure refrigerant stream which provides at least part of each of said first and second coolant means, said combined stream being thereafter passed to said compressor means for recompression and said first and second streams of compressed refrigerant being derived from the recompressed combined stream after said recompressed combined stream has been cooled by said heat exchange with said second coolant means.
 15. A method as claimed in claim 12 in which said Joule-Thomson expansion is effected in at least two steps whereby said second stream of low pressure refrigerant is provided in the form of a plurality of separate flows at different pressures.
 16. A method as claimed in claim 15 in which at least one of said flows is at substantially the same pressure as said first low pressure refrigerant stream and is combined therewith to form a combined low pressure refrigerant stream which provides part of each of said first and second coolant means, the remainder of said second low pressure refrigerant stream is at lower pressure and provides another part of each of said first and second coolant means, said remainder being thereafter compressed to substantially the pressure of said combined low pressure refrigerant stream and then combined therewith and the stream so produced being recompressed to form a stream from which said first and second streams of compressed refrigerant are derived after said recompressed stream has been cooled by said heat exchange with said second coolant means.
 17. A method as claimed in claim 12 in which said first and second coolant means each further include a stream of gas resulting from evaporation of the cargo, said gas stream being separate from the gas to be reliquefied.
 18. A method as claimed in claim 12 in which the gas resulting from the evaporation of the ship''s cargo has a fuel value and a portion of said gas is used to fuel the gas turbine, said portion being separate from the gas to be reliquefied.
 19. A method as claimed in claim 18 in which said portion provides a part of each of asid first and second coolant means before it is supplied to said gas turbine.
 20. A method as claimed in claim 7 in which the liquefied gas is liquefied natural gas.
 21. Ship-board apparatus for the reliquefaction of gas resulting from evaporation of a ship''s cargo of liquefied gas, said apparatus including: refrigerant compressing means comprising first and second compressor means arranged for passage of gaseous refrigerant therethrough in series; expansion turbine means operatively connected to one of said first and second compressor means to drive same; hydrocarbon fueled gas turbine means operatively connected to the other of said first and second compressor meaNs to drive same; expansion valve means; and first, second and third heat exchange means; and further including: first conduit means for passing a first stream of compressed refrigerant from said first and second compression means through said first heat exchange means to cool it and then through said turbine expansion means to work expand it to provide a first low pressure refrigerant stream; second conduit means for passing a second stream of compressed refrigerant through said first, second and third heat exchange means in series to cool and condense the stream and then through said expansion valve means whereby to cause partial vaporization of the condensed stream; means for recovering at least a part of the unvaporized portion of the partially vaporized stream; third conduit means for passing a second low pressure refrigerant stream comprising the unrecovered part of the partially vaporized stream in counter-current indirect heat exchange relationship with said second stream of compressed refrigerant in said third heat exchange means; fourth conduit means for passing both said second low pressure refrigerant stream leaving said third heat exchange means and said first low pressure refrigerant stream in counter-current indirect heat exchange relationship with said second stream of compressed refrigerant in said second heat exchange means and thereafter in counter-current indirect heat exchange relationship with said second and first streams of compressed refrigerant in said first heat exchange means and thereafter returning said first and second low pressure refrigerant streams to said refrigerant compressing means; fifth conduit means for feeding gas evaporated from the liquefied gas cargo into admixture with low pressure refrigerant; and sixth conduit means for passing the recovered part of the unvaporized portion of the partially vaporized stream for return to the liquefied gas cargo.
 22. Apparatus as claimed in claim 21 in which said gas turbine is adapted to be fueled by gas evaporated from the liquid gas cargo, said apparatus also including means for feeding a portion of refrigerant recovered from said first compressor means to said gas turbine means as fuel.
 23. Apparatus as claimed in claim 21 in which the expansion valve means is adapted to provide said second low pressure refrigerant stream as a plurality of flows at different pressures.
 24. Apparatus as claimed in claim 21 arranged in three separable modules, the first of which contains one compressor means and the gas turbine means operatively connected to said compressor means to drive same, the second of which includes the second compressor means and the expansion turbine means operatively connected to said second compressor means to drive same, and the third of which includes a cold box containing said heat exchange means and said expansion valve means.
 25. Ship-board apparatus for the reliquefaction of gas resulting from evaporation of a ship''s cargo of liquefied gas, said apparatus comprising: refrigerant compressing means comprising first and second compressor means arranged for passage of gaseous refrigerant therethrough in series; expansion turbine means operatively connected to one of said first and second compressor means to drive same; hydrocarbon fuel gas turbine means operatively connected to the other of said first and second compressor means to drive same; expansion valve means; and first, second and third heat exchange means; and further including: first conduit means for passing a first stream of compressed refrigerant from said first and second compression means through said first heat exchange means to cool it and then through said turbine expansion means to work expand it to provide a first low pressure refrigerant stream; second conduit means for passing a second stream of compressed refrigerant through said first, second and third heat exchange means in series to cool and condense the partiAl vaporization of said condensed stream to form a second low pressure refrigerant stream; third conduit means for passing said second low pressure refrigerant stream through said third heat exchange means; fourth conduit means for passing gas evaporated from the liquefied gas cargo through said third heat exchange means in indirect heat exchange relationship with said second low pressure refrigerant stream and to pass liquid so formed for return to said liquefied gas cargo; and fifth conduit means for passing both said second low pressure refrigerant stream leaving said third heat exchange means and said first low pressure refrigerant stream in counter-current heat exchange relationship with said second stream of compressed refrigerant in said second heat exchange means and thereafter in counter-current indirect heat exchange relationship with said second and first streams of compressed refrigerant in said first heat exchange means and thereafter returning said first and second low pressure refrigerant streams to said refrigerant compressing means.
 26. Apparatus as claimed in claim 25 in which said gas turbine means is adapted to be fueled by gas evaporated from the liquid gas cargo, said apparatus further including conduit means for passing a portion of the evaporated gas from the cargo in indirect counter-current heat exchange with said compressed refrigerant to warm said portion and then to said gas turbine means as fuel.
 27. Apparatus as claimed in claim 25 in which the expansion valve means is adapted to provide said second low pressure refrigerant stream as a plurality of flows at different pressures.
 28. Apparatus as claimed in claim 25 arranged in three separable modules, the first of which contains one compressor means and the gas turbine means operatively connected to said compressor means to drive same, the second of which includes the second compressor means and the expansion turbine means operatively connected to said second compressor means to drive same, and the third of which includes a cold box containing said heat exchange means and said expansion valve means. 